Six Sigma Fundamentals: A Complete Introduction to the System, Methods, and Tools

What does the Y = f(x) expression mean?

In simple mathematical terms, this means that the Y is a function of x. In plain language, it means that changes in the x (i.e., inputs and processes) will determine how the Y (i.e., the output) will turn out. In the six sigma methodology the Y may mean profits, customer satisfaction, strategic goal, efficiency and so on. On the other hand, the x may mean actions that achieve the strategic goals, influences on customer satisfaction, process variables and so on. Another way of thinking about this equation is to think of the Y as the dependent variable and the x as the independent variable.